Many techniques for filtering or otherwise clarifying audio signals rely upon signal to noise ratios (SNRs). An SNR typically employs an estimate of the amount of noise, or power level of noise, in the audio signal.
A variety of audio devices, including state of the art mobile telephones, include a primary microphone that is positioned and oriented to receive audio from an intended source, and a reference microphone that is positioned and oriented to receive background noise while receiving little or no audio from the intended source. The principal function of the reference microphone is to provide an indicator of the amount of noise that is likely to be present in a primary channel of an audio signal obtained by the primary microphone. Conventionally, it has been assumed that the level of noise in a reference channel of the audio signal, which is obtained with the reference microphone, is substantially the same as the level of noise in the primary channel of the audio signal.
In reality, there may be significant differences between the noise level present in the primary channel and the noise level present in the corresponding reference channel. These differences may be caused by any of a number of different factors, including, without limitation, an imbalance in the manner in which (e.g., the sensitivity with which) the primary microphone and the reference microphone detect sound, the orientations of the primary microphone and the reference microphone relative to an intended source of audio, shielding of noise and/or sound (e.g., by the head and/or other parts of an individual as he or she uses a mobile telephone, etc.) and prior processing of the primary and/or reference channels. When the noise level in the reference channel is greater than the noise level in the primary channel, efforts to remove or otherwise suppress noise in the primary channel may result in over suppression, or the undesired removal of portions of targeted sound (e.g., speech, music, etc.) from the primary channel, as well as in distortion of the targeted sound. Conversely, when the noise level in the reference channel is less than the noise level in the primary channel, noise from the primary channel may be under suppressed, which may result in undesirably high levels of residual noise in the audio signal output by noise suppression processing.
The presence of targeted sound (e.g., speech, etc.) into the reference channel may also introduce error into the estimated noise level and, thus, adversely affect the quality of an audio signal from which noise has been removed or otherwise suppressed.
Accordingly, improvements are sought in estimating the differences in noise and speech power levels.